1. Field of the Invention
The present invention relates to an impeller for a centrifugal fan and a centrifugal fan, and more particularly, to an impeller having blades connected by an outer diameter ring and a centrifugal fan including the impeller.
2. Description of the Related Art
A centrifugal fan is widely used for cooling, ventilation and air conditioning of an electrical household appliance, an OA device and an industrial device, for a vehicular blower and the like. There has been known a centrifugal fan including an impeller having a plurality of blades, and an outer diameter ring connected to tip end portions of the plurality of blades at a side of a discharge opening so as to support the blades.
JP-A-2012-47162 discloses a structure of a centrifugal fan including an impeller of an open impeller type in which a ring member is connected to tip end portions of blades. The centrifugal fan uses a bell mouth, and the blade is formed with a protrusion part entering an inner side of an air suction opening so as to suppress deterioration of noise performance.
JP-A-2001-12389 and JP-A-H7-4389 disclose a structure of an impeller having no outer diameter ring.
Specifically, JP-A-2001-12389 discloses an impeller of a multi-blade fan in which a discharge tip end portion of each blade is bent in a rotating direction so as to improve a P-Q characteristics. The impeller is not an open impeller type and has a structure where the blades are sandwiched between upper and lower plates.
JP-A-H7-4389 discloses a structure of a turbo fan in which a part of a blade close to an outer periphery of an impeller in a section of a plane perpendicular to a rotary shaft of the impeller is bent to be perpendicular to an outer periphery edge of the impeller. JP-A-H7-4389 adopts this structure so as to reduce a blowing noise.
FIG. 19 is a plan view showing a related-art impeller for centrifugal fan having an outer diameter ring. FIG. 20 is a side sectional view of the related-art impeller.
A related-art impeller 810 for a centrifugal fan is described with reference to FIGS. 19 and 20. The impeller 810 has a disc-shaped main plate 831, a plurality of blades 851 and a ring-shaped outer diameter ring 861. The main plate 831 is formed with a rotor holder 833 at a center thereof. At a state where a rotor of a motor is arranged at an inner side of the rotor holder 833, the impeller 810 rotates about a shaft 871, which is provided at a center of the rotor holder 833, by a driving force of the motor. The impeller 810 rotates in a direction shown with an arrow R in FIG. 19. Thereby, the impeller 810 discharges a fluid, which is suctioned from the upper, to a side of the impeller 810.
The plurality of blades 851 are arranged along a circumferential direction about the center part of the main plate 831. Each of the blades is a backward inclined blade and is formed such that the blade forms a gentle spiral shape from a center part of the impeller 810, when seen from a plan view.
Each blade 851 is connected to an inner side of the outer ring 861 at its trailing edge portion 851b. The outer ring 861 is connected to upper portions of the trailing edge portions 851b of the respective blades 851, which are spaced upwards from the main plate 831.
An inner diameter of the outer ring 861, an outer diameter of the main plate 831, a height of the blade 851 and a height of the outer ring 861 are set to be about 113 mm, 111 mm, 20 mm and 1 mm, respectively.
In the above impeller 810, since the blades 851 form the spiral shape, the trailing edge portion 851b of the blade 851 and an inner periphery of the outer ring 861 are connected at an acute angle (that is, a small and sharp angle). Specifically, an angle (a connection angle), which is formed between a pressure surface of the blade 851 and an inner surface of the outer ring 861 at the connection part of the blade 851 and the outer ring 861, is an acute angle. Therefore, a following problem would be caused.
That is, in a mold for molding the impeller 810, the connection part of the impeller 810 and the outer ring 861 has a sharp shape of an acute angle. However, the mold having the shape is apt to be fractured and a trouble may be thus caused when mass-producing the impeller 810.